In recent years, in a wind farm where a plurality of wind turbines are set, output power reducing operation (Delta Power Control) for performing operation for deliberately reducing output powers of the wind turbines and securing a reserve capacity for an output power is performed. For example, United States Patent Publication No. 2010/0286835 discloses that a wind speed-output power characteristic corresponding to the output power reducing operation is set in advance, a target output power corresponding to wind speed at each time is acquired using a wind speed-power curve of the wind speed-output power characteristic, and control of wind turbines is performed to attain the target output power.
United States Patent Publication No. 2011/0144816 discloses that, when an output power at a connection point is equal to or smaller than a target output power, an output power of at least one wind turbine is increased to match the output power at the connection point to the target output power.